Type alias can't access enum variants if the enum is a type parameter default

[Aegrithas]

Given an enum and type alias like so:

type A<T = E> = T;

enum E {
    
    V
    
}

I would expect A::V, A::<>::V and possibly A::<_>::V to refer to E::V.

However, these all generate the same error message:

error[E0599]: no associated item named `V` found for type `_` in the current scope
 --> src/main.rs:2:25
  |
2 |     println!("{:?}", A::V);
  |                         ^ associated item not found in `_`

Note that specifying T explicitly (e.g. A::<E>::V) works as expected.

Meta

rustc --version --verbose:

rustc 1.71.1 (eb26296b5 2023-08-03)
binary: rustc
commit-hash: eb26296b556cef10fb713a38f3d16b9886080f26
commit-date: 2023-08-03
host: x86_64-pc-windows-msvc
release: 1.71.1
LLVM version: 16.0.5

I also got the same behavior on stable, beta and nightly in the playground.

[fmease]

Generic parameter defaults are not used as a fallback when type inference fails. This is a known limitation.
The feature default_type_parameter_fallback (tracking issue, RFC) has never been properly implemented and it has stalled.

Related / duplicate issues: #83687, #96300, #98931 (, #27336).

In your case, <A>::V would force the default type to be used (admittedly this is awkward).

@rustbot label -needs-triage T-compiler A-inference